This invention relates to polymer compositions and their preparation, and more particularly to the preparation of cross-linked polycarbonates.
Polycarbonates are a class of well-known, commercially available thermoplastic materials possessing physical and chemical properties which are useful in a wide variety of applications. Some of the notable attributes of polycarbonates include high impact strength and thermal stability, along with good transparency. However, the use of polycarbonates in some applications, e.g., automotive, is limited somewhat because of their relatively poor resistance to various organic solvents and other chemicals. Furthermore, glass transition and heat distortion temperatures for some of the conventional polycarbonates are not high enough to permit reasonable molding cycle times.
One method for correcting these deficiencies involves cross-linking the polycarbonate chains to form thermoset compositions. For example, U. S. Pat. No. 3,098,056 describes the reaction of epoxy resins with linear polycarbonates and hardeners, while U.S. Pat. No. 3,261,808 describes the preparation and cure of polycarbonates containing epoxy end groups. While polycarbonates cross-linked in this manner have better properties than those of polycarbonate alone, these methods may not be suitable for use in some of the more advanced molding techniques. For example, the high melt viscosities of linear polycarbonates make these methods unsuitable for use under reactive processing conditions such as reaction injection molding (RIM).
A recent development in the area of polycarbonates involves cyclic polycarbonate compositions. The preparation and use of cyclic polycarbonates have been previously disclosed in numerous applications filed for inventors on behalf of the assignee of the present invention. For example, these materials are described in U.S. Pat. No. 4,644,053. As described in this and in other references, the cyclic oligomer mixtures have low viscosities and can be simultaneously polymerized and molded upon the application of heat.
Thermoset compositions prepared by reacting cyclic polycarbonate oligomers with polyepoxy compounds are described in application Ser. No. 019,153 of T. Evans et al., filed Feb. 25, 1987, now U.S. Pat. No. 4,746,725, and assigned to the assignee of the present invention. Although various polyepoxides are described in that application, the material of choice is either a triglycidyl isocyanurate or a bis-epoxy-terminated bisphenol A-epichlorohydrin condensate.
While the materials described by T. Evans et al. are suitable for a wide variety of applications, there is continuing interest in developing cross-linked polycarbonates which are thermally stable at high processing temperatures, e.g., above about 280.degree. C.
It is therefore a primary objective of the present invention to provide an improved method for the preparation of highly cross-linked polycarbonates.
It is another objective of this invention to provide a method for preparing cross-linked polycarbonates which are stable at high processing temperatures.
It is a further objective of the present invention to provide a cross-linked polycarbonate preparation method which is amenable to reactive processing conditions such as reaction injection molding.
It is still another objective to provide thermoset polycarbonate compositions characterized by high density cross-linking.